1. Field of the Invention
The present invention relates to injection molding machines, and in particular to hydraulic drive means for the opening and closing of the two halves of a molding die into which plastic or metallic raw material is injected. More specifically, the present invention relates to hydraulic cylinder operated die closing units of injection molding machines in which the cylinder rods extend in the direction of the opening and closing motion of the movable die plate, which latter may be directly attached to the cylinder rods.
2. Description of the Prior Art
A prior art hydraulic drive for a die closing unit of the type mentioned above is disclosed in my U.S. Pat. No. 3,663,140. This drive consists of at least two parallel hydraulic power cylinders to which two auxiliary cylinders are coaxially connected. While a comparatively moderate force is normally sufficient to separate and approach the die halves, a much larger force is required to keep the die closed during the injection process. The known device takes advantage of this difference, by using the smaller auxiliary cylinders to produce the opening and closing travel of the unit, and using the power cylinders only to create the closing pressure. Under certain special circumstances, the power cylinder is also used to assist in the initial portion of the opening motion, when the molded part offers a resistance against die opening.
In the prior art device, this is accomplished by arranging the pistons of the power cylinders so as to operate both as pistons and as bypass valves which, when open, permit the power piston to execute the opening and closing travel, without removing all the fluid from one side of the power cylinder and simultaneously taking in a comparable amount of fluid on the other side of the cylinder. Instead, the hydraulic fluid simply flows through bypass channels arranged inside the power piston from one side of the piston to the other, as a result of the valve action of the piston. At the end of the closing travel, for example, the power piston valve is closed, whereupon the effective area of the power piston, subjected to the full fluid pressure, exerts its maximum force against the die halves as a closing pressure.
In the above-mentioned prior art arrangement, the power piston itself is the moving part of this internal bypass valve. The valve seat is provided in the form of an upstanding radial shoulder on the piston rod, the piston being seated against this shoulder in the closed valve position. This design has certain shortcomings, among them the material and machining costs of the piston rods with their radial shoulders, the manufacturing and assembly costs of the axially movable power pistons with their internal flow chamber and valve closing face. Another disadvantage relates to the considerable weight of the piston which, being the moving valve body, causes the latter to respond comparatively slowly to a reversal of movement direction, and which also creates a rather hard impact, when the power piston moves against its valve seat. Lastly, the entire closing pressure is transmitted from the power piston to the piston rod through the valve seat, meaning that the valve seat has to be comparatively large in area, with the result that the flow through the open valve seat is more restrained and not as straight and even as would be desirable.